This invention relates to signal processing of multi-bit digital information signals, particularly processing of such information signals which requires conversion to the analog domain.
In the field of information signal processing, wherein human speech, musical sounds or other intelligence is communicated from one location to another in the form of electrical signals, recent trends have favored the use of digital signals over the more traditional analog signals due to several advantages inherent in the digital signal format, such as greater reliability, the ability to pack more information onto a given communication link, greater immunity to interference and other well known advantages. Nevertheless, in spite of the advantages afforded by use of the digital signal format, analog signal processing techniques are regularly employed at critical stages in information transmission systems due primarily to the vast body of analog processing technology which has been developed during the past few decades, which has resulted in thoroughly studied and tested methods and apparatus for performing certain information processing functions.
In the field of telecommunications, for example, and specifically in telephone systems, a time division multiplexed, pulse code modulated digital information signal format has been adopted in recent years for transmitting information from point to point. However, the digital information signals used to convey signaling information, such as the successive identifying digits of a called station, which are usually termed dual tone multiple frequency (DTMF) signals, are typically processed in the analog domain. This is typically accomplished by converting the digital DTMF signals to equivalent analog signals, passing the analog counterpart signals through a pair of parallel connected band pass filters each having a relatively narrow pass band for filtering out all signals except those lying in one of two well defined frequency ranges (i.e., the high band and the low band filters), and detecting the high band frequency component and the low band frequency component by means of suitable detection devices. Successful operation of this analog processing technique is dependent upon the use of high quality band pass filters, the cost of which increases with increasingly exacting performance characteristics, such as shorter minimum response time. For a time division multiplexed digital telephone system having N channels requiring DTMF processing, N high band filters and N low band filters are required, for a total of 2N. In addition, there are additional circuit elements employed in a DTMF analog signal processing system, such as a sample and hold circuit coupled between the digital-to-analog converter and the paired band pass filters which also must be included when expanding the number of channels, which also adds additional costs with increasing channel capacity.
Similar considerations apply to other analog processing techniques in the field of telecommunications, all due to the requirement that the information handling capability of existing communications links and links planned for the future be greatly increased. For example, while pulse code modulation has become the standard digital encoding technique employed in the telephonne industry, other more efficient digital encoding schemes, such as delta modulation, adaptive delta modulation, and continuously variable slope delta modulation, are being increasingly employed in order to convey more information over existing communications highways, such as telephone lines, microwave links and satellite communication links. The great majority of these more efficient digital encoding techniques require processing in the analog domain, using high quality analog processing circuitry. Similarly, echo suppression, echo cancellation and low speed modem communication applications traditionally employ analog processing techniques. Although it is possible to share some of the analog circuitry among some of the multiplicity of information channels, in all known applications a substantial number of high quality, costly analog processing devices have been found to be required.